Elucidation of how the size of a developing embryo is genetically regulated is important because the final volume of endosperm as a storage organ of starch and proteins is affected by embryo size in cereal crops. Researchers have found that genes involved in embryo size contribute to the regulation of endosperm development. Investigation of these genes is important for agriculture because cereal endosperms are the staple diet in many countries.
Rice mutants, having normally differentiated shoot and radicle and either reduced or enlarged embryo when compared to wild type rice, were identified in the early 1990s in plants obtained from methyl-nitrosourea mutagenized Taichung 65 cultivar. Mutant plants displaying an enlarged embryo were designated giant embryo (ge) mutants while plants displaying a smaller embryo were designated reduced embryo (re) mutants (Kitano et al. 1993, Plant J. 3:607-610; Hong et al. in 1995, Dev. Genet. 16:298-310).
The phenotypes of each of the three reduced embryo mutants were designated re1, re2, and re3 even though the gene(s) responsible for these phenotypes have not been characterized. A mutation in a different locus is responsible for the mutant phenotype. Phenotypic analysis of ge and re mutant plants led to the theory that embryo size may be determined by the interaction between embryo-specific genes and endosperm-specific genes regulating endosperm development (Hong et al. (1996) Development 122:2051-2058).
The reduced embryo size phenotype of re2 mutant plants is associated with the enlargement of the endosperm size without altering the overall seed size. This phenotype is potentially useful for improving cereal quality by increasing the amount of endosperm tissue, which is rich in starch and other nutrients. Moreover, the reduction of embryo size in seed has a potential benefit for some milling processes, where embryonic tissues are considered as waste, such as in the production of ethanol.